The correct option is (a) Mg^2+
Easy explanation: Electrophilic catalysis involves covalent intermediate transition state formation between the cationic electrophilic group in the enzyme and the electron rich portion of the substrate. Most of the amino-acid side chains in the active site don’t act as an efficient electrophile. So, most of the enzymes require metal ions such as Mg^2+, Zn^2+, Mn^2+, etc., as an electron deficient electrophile for optimum catalysis. Ser 195 is an amino-acid in the active site chymotrypsin which hydrolyses peptides.